Conventionally, there has been known a power converter that includes a transformer between two conversion circuits for performing power conversion, as a kind of power converter that performs bidirectional DC/DC conversion, or performs bidirectional DC/AC conversion (e.g., refer to JP 2009-177940 A (hereinafter, referred to as “Document 1”)).
As shown in FIG. 12, the power converter in Document 1 includes a transformer 401 between a primary-side DC/AC conversion part 101 and a secondary-side DC/AC conversion part 201. In addition, the power converter has a configuration that a boosting circuit 301 is connected with the secondary-side DC/AC conversion part 201. This power converter is configured to bidirectionally perform DC/DC power conversion. Each of the primary-side DC/AC conversion part 101, the secondary-side DC/AC conversion part 201 and the boosting circuit 301 is configured to perform bidirectional power conversion.
The configuration described in Document 1 has a function that suppresses a reduction of a voltage value between a battery B11 having a low nominal voltage and a battery B12 having a high nominal voltage, by supplying energy to one of the batteries from the other when a voltage value of the one is reduced. In Document 1, the boosting circuit 301 is connected with the secondary-side DC/AC conversion part 201, because the battery B11 having the low nominal voltage is connected with the primary-side DC/AC conversion part 101, and the battery B12 having the high nominal voltage is connected with the boosting circuit 301.
The transformer 401 has a turn ratio between the numbers of turns of windings, which is set according to a voltage ratio between voltages of the batteries B11 and B12. Accordingly, voltage values of the batteries B11 and B12 fluctuate. Therefore, if the turn ratio of the transformer 401 has been set according to the voltage ratio at voltage step-down, it may be impossible to obtain the voltage ratio at voltage step-up with only the transformer 401. In order to solve the shortage of a voltage value at the voltage step-up, the boosting circuit 301 is installed. Furthermore, Document 1 discloses that the boosting circuit is connected with a load, instead of the battery B12.
The circuit referred to as the boosting circuit 301 functions as a boosting chopper circuit at the voltage step-up of supplying energy from the battery B11 to the battery B12, and as a step-down chopper circuit at the voltage step-down of supplying energy from the battery B12 to the battery B11.
Now, as shown in FIG. 2, it is assumed that the nominal voltage of the battery B11 is selected from a relatively-wide range as a characteristic T1, and a range in which the voltage value of the battery B12 varies is relatively-narrow as a characteristic T2. In FIG. 2, a range between top and bottom ends of each vertical bar corresponds to a voltage range. Here, it is assumed that the range of the voltage value of the battery B12 is within a range of the selectable voltage value for the battery B11. As a case where such a condition is satisfied, for example it is considered that the battery B11 is a storage battery installed in a motor vehicle such as an electric vehicle, and the battery B12 is a storage battery for supplying electric power to facility apparatuses to be used in a home.
In the configuration described in Document 1, since the boosting circuit 301 is operated as a boosting chopper circuit upon supplying energy from the battery B11 to the battery B12, the maximum value of an input voltage of the boosting circuit 301 needs to be set lower than the voltage value of the battery B12. In other words, a relatively-large current flows through an input side of the boosting circuit 301, and therefore there is a problem that a relatively-large loss occurs as the whole of the power converter.
On the other hand, since the power converter in Document 1 can perform bidirectional power conversion, it is considered that the voltage values of the batteries B11 and B12 are set to have a relation opposite to that shown in FIG. 2. In other words, it is considered that the voltage value of the battery B11 is selected from a relatively-narrow range, and the range of the selectable voltage value for the battery B11 is within a range of the selectable voltage value for the battery B12.
In this configuration, since the boosting circuit 301 is operated as a step-down chopper circuit upon supplying energy from the battery B12 to the battery B11, the voltage value output from the boosting circuit 301 becomes lower than the voltage value of the battery B11. In other words, the transformer 401 needs to perform the voltage step-up toward the primary-side DC/AC conversion part 101 from the secondary-side DC/AC conversion part 201.
Since the secondary-side DC/AC conversion part 201 is operated at a relatively-low voltage value stepped-down by the boosting circuit 301, a relatively-large current flows through the windings of the transformer 401, and a relatively-large loss may occur as the whole of the power converter.